Recessive mutations in any of six genes (
smg-1 thru
smg-6) result in the suppression of particular mutations in
lin-29,
unc-54, similarity is present in two cases where suppression has been examined at the molecular level: unc- 54, the body-wall muscle myosin heavy chain, and
lin-29, a putative transcription factor required for the expression of adult seam cell fates.
unc-54(
r293) is a small deletion in the 3' untranslated region that removes the polyadenylation signal sequence. The result is a transcript aberrantly large in size, present in greatly reduced amounts. In
unc-54(
r293);smg double mutants, substantially more of the aberrant transcripts are present, presumably permitting sufficient
unc-54 product to be synthesized for near-normal motility (Pulak and Anderson, WBG 11(1):54). However,
lin-29(
n546) is a point mutation in the translated region, converting and Arginine to an opal stop codon, and there does not appear to be a large difference in the size or abundance of
lin-29 transcripts in
n546 or the
n546;smg double mutants vs. wild type ( Rougvie and Ambros, WBG 11(3) :37). Nevertheless,
n546;smg double mutants show complete, allele-specific suppression of the
lin-29 phenotype. This phenomenon may reflect a relatively subtle difference in
lin-29 mRNA accumulation, perhaps due to instability potentiated by a paucity of ribosomes 3' of the opal stop codon. Numerous alternate explanations of the above results are possible, including that both
unc-54(
r293) and
lin-29(
n546) are suppressed at some other step in the transcript to product pathway, and that a process downstream of
lin-29 action, but requiring
n546 product, is stabilized. In order to address the burning question of what the smg genes really do, we are cloning
smg-4 from the contig between
her-1 and
myo-3. We also request alleles of any cloned or about-to-be- cloned genes which show suppression or enhancement in the presence of smg mutations. It is worth checking mutants for smg-suppressibility, since it is often nice to have a homozygous mutant which can be maintained as nearly wild-type and because once the nature of smg- suppression is understood, it may shed light on the expression of suppressible genes as well.